TRK-820 is a novel 4,5 epoxy-morphinan derivative which was designed and intended for use as an analgesic. Early clinical trials using healthy volunteers demonstrated an unexpected aquaretic effect. Since then, the development of TRK-820 has been extended to include a consideration of the utility of the drug in the treatment of hyponatremia associated with fluid overload. The primary objectives of this research are to evaluate the effectiveness and safety of TRK-820 in patients with hyponatremia coincident with New York Heart Association Class III or IV heart failure. Subjects will be administered an intravenous dose of TRK-820 or placebo once each morning for 5 days. Blood sampling, urine collections, and hemodynamic measurements will be performed throughout each study day.